Small diameter boreholes, such as are commonly employed in mining programs, are subject to unpredictable wanderings along their length. For strategic and economic reasons, it is often vital to determine their true trajectories in the subsurface. Only in this way is it possible to obtain a valid three-dimensional picture of the subsurface geology, and, in particular, the true parameters of any mineral deposit whose economic potential is being evaluated.
Various approaches have been made to date to provide the means for determining borehole trajectories. These approaches include, for example, the following: gyroscopes, bending of light in a long tube, tiltmeter and compass and combined tiltmeter and three component magnetometer devices.
Gyroscope (either spinning masses or interferometric) devices measure (and integrate) the rate of deviation of two orthogonal gyros in a sonde to obtain the orientation of the sonde at any point in the borehole. Typical examples of such a device include the BGO-1 gyro orientation probe manufactured by the IFG Corporation, Brampton, Ontario; the RGS-OR rate gyro manufactured by Gyrodata, Houston Tex.; or the MT9 sensor manufactured by Xsens, Enschede, The Netherlands.
With respect to bending of a long tube, these devices typically use a collimated light beam, projected from a source at one end of a long, flexible tube and a target at the other end. The deviation of the incident light beam from the centre of the target (up-down and left-right) is measured and recorded, to determine the local curvature of the borehole in the vertical and horizontal planes. These curvatures are integrated, to determine the orientation of the borehole at any point. An example of this type of orientor is the Fotobor, manufactured by Reflex Instruments Limited, Sweden.
With respect to tiltmeter and compass devices, these devices feature a mechanical tiltmeter and a mechanical compass in a sonde. The status of both of these mechanical devices may be photographed at points down the borehole, using a miniature camera in the sonde. In a simpler (single shot) version, the two devices are clamped after an elapsed time, at one station. The Pajari instrument, manufactured by Pajari Instruments, Inc., Orillia, Ontario, is an example of this latter type of device.
The combined tiltmeter and three component magnetometer device employs a three component magnetometer and dual axes tiltmeters in a sonde to derive the borehole orientation. The BVM-01 vector magnetometer borehole probe manufactured by IFG Corporation, is an example of such a device. The BVM-01 device, however, requires complex instrumentation. Firstly, the BVM-01 device requires the use of two tiltmeters, one to measure the inclination of the sonde relative to the vertical and the other to measure the angle of rotation of the long axis of the sonde relative to the vertical. Secondly, the BVM-01 device requires the use of three magnetic sensors, measuring three mutually orthogonal components of the Earth's magnetic field.
All of the above devices suffer from one or more limitations, such as high capital cost, cumulative errors, drifts, instrument complexity or inefficiency in operation. It is therefore an object of the present invention to provide a novel method and apparatus for determining the orientation of a borehole at a point along the borehole, which is simpler in instrumentation and lower in cost than the above approaches.